Automatic screw machines with single spindle and multiple spindle lathes are provided with a bar stock feeding apparatus which feeds bar stock to the machine. Bar stock can be formed in a variety of shapes and sizes. The most common type of bar stock is circular in shape. Square and hexagonal bar stock may also be machined in automatic screw machines. However, most conventional bar stock retaining mechanisms cannot be easily adapted to engage and feed different shapes and sizes of bar stock.
Two common clamping arrangements are used for automatically engaging and feeding bar stock to an automatic screw machine. The first type of clamping arrangement comprises an endless belt and pulley arrangement for feeding bar stock. An example of this type of arrangement is disclosed in U.S. Pat. No. 4,417,491 issued to Uehara et al., which discloses an apparatus for automatically feeding bar stock material into a machine tool with a plurality of bar stock clamping members. Each bar stock clamping member has a pair of bar engaging jaws with a set of guide pulleys and a drive pulley. An endless belt connects the guide pulleys and drive pulley to create an isosceles triangle gripping arrangement. The pair of jaws is rotatably mounted on a swingable arm which is attached to a support plate. The clamping jaws are rotated into contact with the bar stock to keep the bar stock aligned with the spindle of the lathe. This arrangement is particularly susceptible to mechanical breakdowns and endless belt failures.
The second type of clamping arrangement consists of a movable clamping portion and a fixed clamping guide portion. U.S. Pat. No. 4,292,864 to Cucchi et al. discloses an automatic feeder for a multi-mandrel lathe. The '864 patent provides a plurality of clamping members having a fixed clamping element and a movable clamping element. Each clamping element has an elastomeric coating on the interior surface. The movable clamping element is pivotally attached to an adjacent fixed clamping element of another clamping pair by a hinge. The movable clamping element pivots about the hinge to open to receive a length of bar stock.
The arrangement disclosed in the '864 patent is not an efficient method of clamping bar stock when a multiple spindle lathe is operated at high speeds. Changing bar stock using this arrangement may cause machine down time of up to 30%. Also, if one of the clamps is damaged during operation, the entire clamping arrangement must be replaced. In addition, the clamping arrangement disclosed in the '864 patent does not positively lock the clamping portions around the bar stock. Bar stock may work loose in such a clamping arrangement because the clamp is not positively locked.
Another problem associated with the '864 patent is loading and feeding bar stock into the clamping members. Sections of bar stock are typically are 8 to 12 feet in length and may range in weight to over 200 pounds. The arrangement in the '864 patent requires the operator to load the bar stock at the top of the bar feeding apparatus so the bar stock can be gravity loaded into the clamping members. This loading arrangement is unwieldy and requires heavy loads to be lifted 4 or more feet off the ground.